B cells in the pneumococcus-infected lung are heterogeneous and require CD4⁺ T cell help including CD40L to become resident memory B cells

Recovery from respiratory pneumococcal infections generates lung-localized protection against heterotypic bacteria, mediated by resident memory lymphocytes. Optimal protection in mice requires re-exposure to pneumococcus within days of initial infection. Serial surface marker phenotyping of B cell populations in a model of pneumococcal heterotypic immunity revealed that bacterial re-exposure stimulates the immediate accumulation of dynamic and heterogeneous populations of B cells in the lung, and is essential for the establishment of lung resident memory B (B_RM) cells. The B cells in the early wave were activated, proliferating locally, and associated with both CD4⁺ T cells and CXCL13. Antagonist- and antibody-mediated interventions were implemented during this early timeframe to demonstrate that lymphocyte recirculation, CD4⁺ cells, and CD40 ligand (CD40L) signaling were all needed for lung B_RM cell establishment, whereas CXCL13 signaling was not. While most prominent as aggregates in the loose connective tissue of bronchovascular bundles, morphometry and live lung imaging analyses showed that lung B_RM cells were equally numerous as single cells dispersed throughout the alveolar septae. We propose that CD40L signaling from antigen-stimulated CD4⁺ T cells in the infected lung is critical to establishment of local B_RM cells, which subsequently protect the airways and parenchyma against future potential infections.